Glaucoma, a serious long-term health care problem, is a disorder of the eye in which elevated intraocular pressure ultimately leads to damage to the optic nerve and to blindness. Glaucoma has been cited as the second most common cause of blindness affecting several million people in the United States alone.
In order to fully appreciate the present invention, a brief overview of the anatomy of the eye is provided. As schematically shown in FIG. 1, the outer layer of the eye includes a sclera 17 that serves as a supporting framework for the eye. The front of the sclera includes a cornea 15, a transparent tissue that enables light to enter the eye. An anterior chamber 7 is located between the cornea 15 and a crystalline lens 4. The anterior chamber 7 contains a constantly flowing clear fluid called aqueous humor 1. The crystalline lens 4 is connected to the eye by fiber zonules, which are connected to the ciliary body 3. In the anterior chamber 7, an iris 19 encircles the outer perimeter of the lens 4 and includes a pupil 5 at its center. The pupil 5 controls the amount of light passing through the lens 4. A posterior chamber 2 is located between the crystalline lens 4 and the retina 8.
As shown in FIG. 2, the anatomy of the eye further includes a trabecular meshwork 9, which is a narrow band of spongy tissue that encircles the iris 19 within the eye. The trabecular meshwork has a variable shape and is microscopic in size. It is of a triangular cross-section and of varying thickness in the range of 100-200 microns. It is made up of different fibrous layers having micron-sized pores forming fluid pathways for the egress of aqueous humor. The trabecular meshwork 9 has been measured to about a thickness of about 100 microns at its anterior edge, Schwalbe's line 18, which is at the approximate juncture of the cornea 15 and sclera 17.
The trabecular meshwork widens to about 200 microns at its base where it and iris 19 attach to the scleral spur. The passageways through the pores in trabecular meshwork 9 lead through very thin, porous tissue called the juxtacanalicular trabecular meshwork 13 that in turn abuts the interior side of a structure called Schlemm's canal 11. Schlemm's canal 11 is filled with a mixture of aqueous humor and blood components and branches off into collector channels 12 which drain the aqueous humor into the venous system. Because aqueous humor is constantly produced by the eye, any obstruction in the trabecular meshwork, the juxtacanalicular trabecular meshwork or in Schlemm's canal prevents the aqueous humor from readily escaping from the anterior eye chamber which results in an elevation of intraocular pressure within the eye.
As shown in FIG. 2, the eye has a drainage system for the draining aqueous humor 1 located in the corneoscleral angle. In general, the ciliary body 3 produces the aqueous humor 1. This aqueous humor flows from the posterior chamber 2 through the pupil 5 into the anterior chamber 7 to the trabecular meshwork 9 and into Schlemm's canal 11 to collector channels 12 to aqueous veins. The obstruction of the aqueous humor outflow which occurs in most open angle glaucoma (i.e., glaucoma characterized by gonioscopically readily visible trabecular meshwork) typically is localized to the region of the juxtacanalicular trabecular meshwork 13, which is located between the trabecular meshwork 9 and Schlemm's canal 11, more specifically, the inner wall of Schlemm's canal. It is desirable to correct this outflow obstruction by enhancing the eye's ability to use the inherent drainage system.
When an obstruction develops, for example, at the juxtacanalicular trabecular meshwork 13, intraocular pressure gradually increases over time. Therefore, a goal of current glaucoma treatment methods is to prevent optic nerve damage by lowering or delaying the progressive elevation of intraocular pressure. Many have searched for an effective method of lowering and controlling intraocular pressure. In general, various pharmaceutical treatments have been employed to control intraocular pressure. While these treatments may be effective for a period of time, the intraocular pressure in the diseased eyes often increases in many patients. The most frequent problems result from patients failing to follow their treatment regimen thus causing inadequately controlled glaucoma, which results in irreversible damage to the optic nerve that ultimately results in vision loss.
After a trial of pharmaceutical treatments fails to stop the progression of elevated intraocular pressure, or in some cases as primary therapy, a surgical treatment method or procedure is generally performed on the eyes of the patients. The human eye is a particularly challenging target for corrective surgery because of the size, fragility, distribution and characteristics of interior tissues. Surgical attempts to lower the intraocular pressure include various therapies that generally fall under the name “glaucoma filtering surgery”.
The surgical therapies in current use, however, do not address the location of the outflow obstruction that is recognized for causing the elevated intraocular pressure. These procedures include mechanically cutting portions of the eye anatomy and are known by such names as trabeculectomy, trabeculotomy, goniotomy and goniocurettage. Significantly, these techniques have been found to be unsuccessful for long term intraocular pressure control. Trabeculectomy has been the most popular procedure in glaucoma surgery in which an opening is created in the sclera to enable aqueous humor to drain into channels external to the eye globe. This procedure, however, has many complications including leaks, infections, hypotony (e.g., low eye pressure), and requirements for post-operative needling, undesirable antimetabolite use, a need for flap suture adjustment to maintain the function of the opening and a need for long-term monitoring to avoid late complications. Another procedure, called deep sclerectomy, attempts to create an intrascleral filtration pocket, but does not alter anatomic relationships and does not treat the region of outflow obstruction. Another procedure, called viscocanalostomy, does attempt to alter the outflow obstruction between Schlemm's canal and the porous juxtacanalicular layer. In viscocanalostomy, an opening via the sclera is created in an attempt to localize and insert a tube into Schlemm's canal without puncturing the trabecular meshwork. Schlemm's canal is dilated by injection of viscoelastic materials into the canal. By altering the juxtacanalicular meshwork's anatomic relationships, an increased aqueous outflow results. Although attempting to address the outflow obstruction that causes the increased intraocular pressure, viscoanalostomy has not been shown to be successful. Thus, a new effective treatment method was needed for glaucoma to address the outflow obstruction that causes elevated intraocular pressure.
In the prior art, lasers have been used to treat glaucoma. Specifically, lasers have been used to thermally modify and/or to puncture completely through various structures, including the trabecular meshwork, Schlemm's canal and the sclera. Moreover, lasers have been used in attempts to open the anterior chamber to an internal outflow rather than an external outflow channel, or reservoir. Early attempts utilized the lasers available at that time which included Q-switched ruby lasers, neodymium:yttrium aluminum garnet (Nd:YAG) lasers, and argon lasers. These procedures had many names: laser trabeculopuncture, laseropuncture, goniopuncture, laser trabeculostomy, laser trabeculotomy, and laser trabeculoplexy. The above described procedures attempted to remove or move or alter portions of the trabecular meshwork. The procedures have several shortcomings. First, they have limited ability to lower the intraocular pressure to a desirable level. Second, while most found initial success in creating a puncture through the meshwork, the short duration of the reduced intraocular pressure proved to be ineffective in treating the long term effects of glaucoma. As a result, patients suffered undesirable additional postoperative procedures to lower the intraocular pressure and required continuous long-term monitoring. The short duration of the reduced pressure has been linked to the body's subsequent inflammatory healing response at the openings created in the eye. The trauma associated with the shearing and tearing of the tissues and the thermal tissue damage caused by the above procedures initiates wound-healing processes which tend, with time, to reseal the created openings.
These early laser procedures failed in that no consideration was given to the size of the openings in the trabecular meshwork. In addition, these procedures also failed to recognize the importance of reducing collateral tissue damage surrounding the created hole. It has been seen that large areas of surrounding tissue damage invite greater inflammation that results in a greater healing response. In addition, if damage occurs to the outer wall of Schlemm's canal and collector channel openings, resultant scarring prevents aqueous humor egress through the distal outflow pathways and effectively eliminates any benefit of the attempted procedure. The actual and potential thermal effect produced by the lasers is a significant contributing factor to the resultant tissue damage. Therefore, the opening size and tissue damage needs to be controlled by controlling the thermal trauma to the target tissues.